The use of light in the ultraviolet-visible-near infrared wavelength range to image and characterize biological tissues is being widely pursued. These efforts have relied on several techniques. A first technique is absorption spectroscopy in which molecules electronically absorb certain wavelengths of light and hence attenuate the transmission or reflectance of that light to yield characteristic "absorption spectra". A second technique is Raman spectroscopy in which molecules vibrationally absorb certain wavelengths of light, more in the infrared, and hence attenuate transmission yielding "Raman spectra". A third technique is fluorescence spectroscopy in which molecules absorb certain wavelengths of light and re-emit longer wavelengths of fluorescence yielding characteristic "fluorescence spectra". A fourth technique is scattering spectroscopy, in which photons of different wavelengths are scattered differently by cells yielding "scattering spectra".
Motivated by a desire to better exploit scattering spectroscopy, this method of imaging concentrates the image contrast mechanism into the upper couple hundred micrometers of tissue. This superficial layer of tissue is the region where tissue pathology arises in many tissues.
One type of light used for imaging of materials is polarized light. Polarized light is strongly reflected off the surface of a material at the air/material interface. This reflectance depends on whether the polarized light is aligned "parallel" or "perpendicular" to the plane of the material. "Parallel" polarized light bounces off the material surface. "Perpendicular" polarized light penetrates into the material. This distinction between parallel and perpendicular alignment of polarized light is the basis of polarized lens in sunglasses which reject the parallel light reflected off a road surface.
Two approaches toward using this distinction between parallel and perpendicular light have been practiced. The first approach involves imaging material surfaces by selective acceptance of parallel polarized light. For example, polarized light has been used to detect "man-made" materials such as glass and metal within a field of "natural" materials such as trees, foliage, and organic soil. The second approach involves imaging material depths by selective rejection of parallel polarized light. For example, polarized light has been used to discriminate the skin surface from the skin depth. Illuminating the skin surface with parallel polarized light and viewing the skin by eye through glasses which are polarized parallel will emphasize the skin surface. Illuminating with parallel polarized light while viewing with glasses that are perpendicular polarized light will emphasize the tissue depth. In the latter case, there is always some parallel light which enters the skin but this light becomes randomly polarized by scattering within the tissue. Hence, viewing through perpendicular polarized glasses essentially rejects the surface reflectance and views the tissue depth with randomly polarized light. Imaging has been described that illuminates with perpendicular polarized light to achieve penetration of light into a tissue, then uses two wavelengths of light to enhance the contrast of a buried vessel based on absorption spectroscopy. Again, the image is based on light that penetrates deeply into the tissue and hence becomes randomly polarized. Viewing through an optical element which selects perpendicular polarized light offers a means of rejecting the glare of surface reflectance.
The task of identifying tissue pathology in the superficial tissue layers, however, is not served by either of the above. About 2-4% of the parallel polarized light is reflected by the surface. Such light does not interrogate the inner tissue where the pathology is located. About 91-93% of the reflected light is randomly polarized and is comprised of light that has penetrated deeply and been multiply scattered by the tissue. Such light is only a blinding artifact while attempting to observe the superficial tissues where pathology arises. Even observing the perpendicularly polarized light component of such multiply scattered deeply penetrating randomly polarized light does not discriminate light that scatters superficially from light that penetrated deeply. Only about 5% of the reflected light is not randomly polarized but is back-scattered by the superficial couple hundred micrometers of tissue. This invention provides a device to image based solely on that 5% of light that has penetrated the surface but not penetrated the tissue depth.